Electron surface collision

Sq —> Sj) electronic transition in the molecule. Intersystem crossing and internal conversion, occurring collision-free in the molecule, carry it onto the ground electronic surface. Collisions of the hot donor molecule, S, with bath molecules, B, depicted by the equation at the bottom of the figure, cause the molecule to lose energy with a probability P E, E ), where A = E -E. 

EM Noise Although Electron Multipliers are a type of pulse counting device, the signal measured from many pulses (that resulting from many secondary ion impacts per second) displays a distribution in current (the number of electrons produced per ion impact). This distribution arises from the statistical nature of the ion to electron conversion process, as well as the processes responsible for the additional electrons formed in subsequent electron-surface collisions. 

A comer-stone of a large portion of quantum molecular dynamics is the use of a single electronic surface. Since electrons are much lighter than nuclei, they typically adjust their wavefiinction to follow the nuclei [26]. Specifically, if a collision is started in which the electrons are in their ground state, they typically remain in the ground state. An exception is non-adiabatic processes, which are discussed later in this section. 

The natural processes of intersystem crossing and internal conversion will quickly (e.g. 50 ns) carry the molecule from this excited electronic surface to the ground electronic surface without a collision,... 

Fig. 4. Accumulating evidence is starting to show that molecules which undergo large amplitude vibration can interact strongly with metallic electrons in collisions and reactions at metal surfaces. This suggests that the Born-Oppenheimer approximation may be suspect near transition states of reactions at metal surfaces.

Tully has discussed how the classical-path method, used originally for gas-phase collisions, can be applied to the study of atom-surface collisions. It is assumed that the motion of the atomic nucleus is associated with an effective potential energy surface and can be treated classically, thus leading to a classical trajectory R(t). The total Hamiltonian for the system can then be reduced to one for electronic motion only, associated with an electronic Hamiltonian Jf(R) = Jf t) which, as indicated, depends parametrically on the nuclear position and through that on time. Therefore, the problem becomes one of solving a time-dependent Schrodinger equation ... 

The role of gas phase initiation processes was further explored by Tibbitt et al. . These authors proposed that the polymerization of unsaturated hydrocarbons in a 13.56 MHz plasma is initiated by free radicals formed in the gas by electron-monomer collisions, the initiation reactions listed in Table 6. Moreover, it was assumed that the formation of free radicals on the polymer surface due to the impact of charged particles could be neglected. This assumption is supported by the fact that at 13.56 MHz and pressures near one torr the discharge frequency is significantly greater than either f, or f and that as a result the fluxes of charged particles to the electrode surfaces are quite small. 

D. A. Micha and E. Q. Feng. The calculation of electron transfer probabilities in slow ion-metal surface collisions. Computer Phys. Comm., 90 242, 1994. 

Incidentally we note that resonances do exist, however, in gas-surface collisions in which, as a consequence of the infinite mass of the solid, J is always zero resonances are indeed one major source of information on the gas-surface interaction (Hoinkes 1980 Barker and Auerbach 1984). Likewise, resonances are prominent features in electron-atom or electron-molecule collisions (Schulz 1973 Domcke 1991) the extremely light mass of the electron implies that only partial waves with very low angular momentum quantum numbers contribute to the cross section. 

Pfandzelter R, Bernhard T, Winter H (2001) Spin-polarized electrons in collisions of multi-charged nitrogen ions with a magnetized Fe(001) surface. Phys Rev Lett 86 4152... 

For 4-atom systems such as formaldehyde the photodissociation dynamics have yet to be established. A fully quantal description of the photodissociation is still very expensive computationally. So far 6-dimensional wave packet studies have been applied only to one electronic surface, for example for scattering of a molecule or atoms on surfaces or collisions of molecules. Currently, for molecular systems with more than four atoms, a reduced dimensionality model must be used. This means that certain degrees of freedom are fixed throughout the dynamical simulation. 

Photoelectron spectroscopy of valence and core electrons in solids has been useful in the study of the surface properties of transition metals and other solid-phase materials. When photoelectron spectroscopy is performed on a solid sample, an additional step that must be considered is the escape of the resultant photoelectron from the bulk. The analysis can only be performed as deep as the electrons can escape from the bulk and then be detected. The escape depth is dependent upon the inelastic mean free path of the electrons, determined by electron-electron and electron-phonon collisions, which varies with photoelectron kinetic energy. The depth that can be probed is on the order of about 5-50 A, which makes this spectroscopy actually a surface-sensitive technique rather than a probe of the bulk properties of a material. Because photoelectron spectroscopy only probes such a thin layer, analysis of bulk materials, absorbed molecules, or thin films must be performed in ultrahigh vacuum (<10 torr) to prevent interference from contaminants that may adhere to the surface. 

Theoretically, if reactions are able to proceed through either a Rideal-Eley step or a Langmuir-Hinshelwood step, the Langmuir-Hinshelwood route is much more preferred due to the extremely short time scale (picosecond) of a gas-surface collision. The kinetics of a Rideal-Eley step, however, can become important at extreme conditions. For example, the reactions involved during plasma processing of electronic materials... 

Fig. 38. The probability to exit on the reactive side (of the upper electronic potential energy surface as a function of the relative velocity in a near collinear CH3I + CH3I collision, see inset. The results are shown for three impact parameters as indicated. The arrow indicates the nominal energy threshold for accessing the upper electronic surface. Computed by the quantal FMS method. The reactive side includes both the formation of molecular products (CH3CH3 +12 as well as CH3 + CH3 + I2 etc.).

Scanning electron microscopy (SEM) is a useful technique for the analysis of plastic surfaces. It involves a finely collimated beam of electrons that sweeps across the surface of the specimen being analyzed. The beam is focused into a small probe that scans across the surface of a specimen. The beam s interactions with the material results in the emission of electrons and photons as the electrons penetrate the surface. The emitted particles are collected with the appropriate detector to yield information about the surface. The final product of the electron beam collision with the sample surface topology is an image (Fig. 10.18). 

Kasemo B 1996 Charge transfer, electronic quantum processes, and dissociation dynamics in molecule-surface collisions Surf. Sci. 363 22... 

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